[AMRadio] AMRadio Digest, Vol 89, Issue 39

[Rob Atkinson]

I was going to post a reply about this to Ed McKie a few days ago and
blew it off because of power fatigue but one of my points was that you
can get a thermocouple amp meter at a fest flea market for not much
(yes I know they may be broken but not _all_ of them are) but if you
play by the nonsensical 375 watt business do the math and you get a
current value that you can't measure accurately with an old analog
meter.   I think it is something like 2.7816 A.   This is why if there
is going to be any kind of AM power limit, it should be input on the
PA that can be measured with the PA current and B+ voltage, or some
nice measurable power output into 50 ohms like 450 watts which would
be 3 A and much easier to nail down.  3.5 A would be better hi hi.

73

Rob
K5UJ

On Tue, Jun 21, 2011 at 4:57 PM, Jim Wilhite <w5jo at brightok.net> wrote:
> There are a couple of wattmeters out there today that is almost in reach
> of the common amateur that exhibit very good accuracy.  Compared to the
> older versions Mirage, Heath and others, they are real good.  One is the
> LP-100A made by N8LP.  It will read the net power out and show the
> reverse power.  In that mode it is close to the power going out of its
> detector.
>
> The thing can be purchased as a kit or wired and calibrated.  If you
> want to build it and verify its operation then send it back, he will
> calibrate it for you.
>
> >From the telepostinc website:
>
> Three autotanging scales with independently adjustable maximums.
> SWR display resolution is .01
> SWR error < .15  (5%) from about 100mW to 3000W, <.05 typical
> Power accuracy is better than 5% at any frequency from ~1W to maximum,
> 3% typical with NIST traceable factory calibration
> Power display is actual power delivered to the load ( Fwd minus Ref
> power) or traditional Fwd power.
>
>
> I don't know about the Coaxial Dynamics version, but I hear it is very
> accurate as well.  I read their description some time back but just
> don't remember.  The problem with either of these is the cost.  The kit
> version of the LP-100A is $335.  That is more than most guys want to pay
> for just an accessory, but if you want to know your peak power, it will
> do that with good accuracy.
>
> The other way is as Don suggests for AM.  Get the good old thermocouple
> RF ammeter and use a scope.  A little math never hurt anyone.
>
> Jim/W5JO
>
> ----- Original Message ----- >
>> The only way to measure real actual transmitter power is to use a true
>> RMS-reading voltmeter or ammeter working into an accurately known
>> resistive
>> load.  One such instrument is the Bird APM-16 (very expensive). A
>> cheaper
>> alternative is the old-fashioned thermocouple RF ammeter. I have tried
>> several of  the popular Hammy Hambone "wattmeters" and never found two
>> that
>> would give the same reading with the same transmitter running the same
>> input
>> power.  The FCC admitted as much when they deleted the former  rule
>> requiring amateurs to have on hand "accurate instruments" to measure
>> transmitting power when running more than 900 watts, then going on to
>> state
>> (as quoted approximately from memory) that "the 'modern day' amateur
>> has
>> 'means other  than accurate measurement' for determining transmitter
>> power".
>
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